Nervous system maps are of critical importance for understanding how nervous systems develop and function. We systematically map here all cholinergic neuron types in the male and hermaphrodite C. elegans nervous system. We find that acetylcholine (ACh) is the most broadly used neurotransmitter and we analyze its usage relative to other neurotransmitters within the context of the entire connectome and within specific network motifs embedded in the connectome. We reveal several dynamic aspects of cholinergic neurotransmitter identity, including a sexually dimorphic glutamatergic to cholinergic neurotransmitter switch in a sex-shared interneuron. An expression pattern analysis of ACh-gated anion channels furthermore suggests that ACh may also operate very broadly as an inhibitory neurotransmitter. As a first application of this comprehensive neurotransmitter map, we identify transcriptional regulatory mechanisms that control cholinergic neurotransmitter identity and cholinergic circuit assembly.DOI: http://dx.doi.org/10.7554/eLife.12432.001
The observation that Tcf3 (MGI name: Tcf7l1) bound the same genes as core stem cell transcription factors, Oct4 (MGI name:Pou5f1), Sox2 and Nanog, revealed a potentially important aspect of the poorly understood mechanism whereby Wnts stimulate self renewal of pluripotent mouse embryonic stem (ES) cells. Although the conventional view of Tcf proteins as the β-catenin-binding effectors of Wnt signaling suggested Tcf3-β-catenin mediated activation of target genes would stimulate ES cell self renewal, here we show that an antagonistic relationship between Wnt3a and Tcf3 on gene expression is important for regulating ES cell self renewal. Genetic ablation of Tcf3 replaced the requirement for exogenous Wnt3a or GSK3-inhibition for self renewal of ES cells, demonstrating that inhibition of Tcf3-repressor is the necessary downstream effect of Wnt signaling. Interestingly, the molecular mechanism underlying Wnt’s effects required both Tcf3-β-catenin and Tcf1-β-catenin interactions, as they each contributed to Wnt stimulation of self renewal and gene expression. Finally, the combination of Tcf3 and Tcf1 was necessary to recruit Wnt-stabilized β-catenin to Oct4 binding sites in ES cell chromatin. These results elucidate the molecular link between the effects of Wnt and the regulation of the Oct4/Sox2/Nanog network.
The dual function of stem cells requires them not only to form new stem cells through self-renewal but also to form lineage-committed cells through differentiation. Embryonic stem cells (ESC), which are derived from the blastocyst inner cell mass, retain properties of self-renewal and the potential for lineage commitment. To balance self-renewal and differentiation, ESC must carefully control the levels of several transcription factors, including Nanog, Sox2, and Oct4. While molecular mechanisms promoting transcription of these genes have been described, mechanisms preventing excessive levels in self-renewing ESC remain unknown. By examining the function of the TCF family of transcription factors in ESC, we have found that Tcf3 is necessary to limit the steady-state levels of Nanog mRNA, protein, and promoter activity in self-renewing ESC. Chromatin immunoprecipitation and promoter reporter assays showed that Tcf3 bound to a promoter regulatory region of the Nanog gene and repressed its transcriptional activity in ESC through a Groucho interaction domaindependent process. The absence of Tcf3 caused delayed differentiation of ESC in vitro as elevated Nanog levels persisted through 5 days of embryoid body formation. These new data support a model wherein Tcf3-mediated control of Nanog levels allows stem cells to balance the creation of lineage-committed and undifferentiated cells.
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